Industrial coatings are surface protective coatings (paint coatings) applied to substrates and typically cured or crosslinked to form continuous films for decorative purposes as well as to protect the substrate. A protective coating ordinarily comprises an organic polymeric binder, pigments, and various paint additives, where the polymeric binder acts as a fluid vehicle for the pigments and imparts rheological properties to the fluid paint coating. Upon curing or crosslinking, the polymeric binder hardens and functions as a binder for the pigments and provides adhesion of the dried paint film to the substrate. The pigments may be organic or inorganic and functionally contribute to opacity and color in addition to durability and hardness. Protective coatings which contain little or no opacifying pigments are described as clear coatings. The manufacture of protective coatings involves the preparation of a polymeric binder, mixing of component materials, grinding of pigments in the polymeric binder, and thinning to commercial standards.
Epoxy resins are particularly desirable for use in protective surface coating materials as a vehicle or polymeric binder for the pigments, fillers, and other additives where the epoxy resins advantageously provide toughness, flexibility, adhesion, and chemical resistance. Water-dispersed coating compositions containing epoxy resins are highly desirable for can coating compositions and particularly useful for interior surfaces of containers. Coatings for soft drink and beer cans, for instance, are critical due to taste sensitivity wherein such can coatings must not alter the product taste of beverages in the containers. Taste problems can occur in a variety of ways such as by leaching of coating components into the beverage, or by absorption of flavor by the coating, or sometimes by chemical reaction, or by perhaps some combination thereof.
Container coating technology frequently utilizes an epoxy resin which has been grafted with acrylic monomers, styrene, and methacrylic acid. This grafted epoxy resin is prepared in solvent, usually butyl cellosolve, and n-butanol, to maintain low processing viscosities and then reduced with water by a direct or inverse let down procedure. Although cured film properties are highly desirable, such coatings suffer from the fact that sizeable amounts of solvents are required to obtain good performance. High molecular weight epoxy resins typically require 25% to 50% solvent (based on total solids plus organic solvent) before reducing with amine and water.
Epoxy based can coatings comprising a carbon grafted acrylic chain are disclosed is commonly assigned U.S. Pat. No. 4,212,781 which teaches a carbon grafting process involving solvent polymerization at moderate temperatures with high levels of peroxide initiator to produce a carbon-graft polymer. The high solvent levels, however, invariably carry over to the aqueous dispersion when the resulting polymers are dispersed into water to produce a VOC (volatile organic compounds) level considerably above 2 and typically between 3 and 4 pounds volatile organic compounds per gallon of resin solids. The acrylic grafted epoxy is particularly useful when utilized with a coreactive crosslinking melamine crosslinker,
Aqueous coating compositions based on microgel resin reaction product obtained by the esterification reaction of epoxy resin with carboxyl group containing vinyl polymer are disclosed in U.S. Pat. No. 4,897,434 where major amounts of high molecular weight epoxy are esterified in organic solvent with the carboxyl vinyl polymer to produce a non-gelled epoxy ester. The epoxy ester is subsequently dispersed into water followed by further coreacting of available epoxy and carboxyl groups on the preformed epoxy ester to form a microgel product. In commonly assigned U.S. Pat. No. 5,508,325, aqueous dispersed microgel polymers are produced by dispersing carboxyl functional acrylic-epoxy copolymer resin into water followed by dispersion of diepoxide and then crosslinking of the carboxyl copolymer by the diepoxide.
It now has been found that excellent aqueous dispersed protective coating compositions exhibiting improved film integrity properties can be prepared based on a polymeric binder comprising an aqueous dispersed carboxyl functional latex polymer combined with an acrylic-epoxy copolymer and subsequently crosslinked with low molecular weight diepoxide to form aqueous dispersed microgel polymer particles. In this invention, preferred latexes are first swollen with minimal additions of base such as amine or ammonia prior to crosslinking with the diepoxide. In a preferred aspect of this invention, the swollen carboxyl functional latex is mixed with an aqueous dispersed acrylic-epoxy carboxyl functional coploymer, whereupon the liquid diepoxide is added to the resulting polymeric mixture and heated for sufficient time to permit crosslinking between the carboxyl functionalities and the diepoxide resin. Swollen latex polymers provide more body and increase viscosity of the polymeric mixture along with high molecular weight while maintaining desired rheology control and achieving very low VOC coatings. By using latex polymers, much higher molecular weight polymers can be utilized while the VOC can be considerably reduced. Minor amounts of acrylic-epoxy copolymer enables dispersion of the diepoxide into the latex polymer. The paint films produced from the polymeric microgel polymer particles exhibit excellent water resistance along with clear glossy protective films. When the paint films are baked, the dispersion polymers further react where the microgel polymer particles cure together by an esterification reactions to yield a tough, water resistant film. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and the illustrative examples.